Seasonal variability of mineral composition of suspended material in the Yangtze River
Thursday, 18 December 2014
Water discharge and suspension load of a river are potentially recorded in sediments in the drainage and / or the river mouth, which could provide us useful proxies for paleoclimatic study. Sediment load from the Yangtze River to the East China Sea (ECS) from the delta to the Okinawa Trough have been widely used to reconstruct the East Asian summer monsoon (EASM) in the past since the water discharge from the Yangtze would be highly affected by monsoon rain, which could deliver much fresh water and sediment to the ECS. Theoretically, sediment provenance and its yield could be changed from time to time depending on the distribution of precipitation which would control the balance of water discharges from the tributaries. Therefore, we need to know the sediment budget along the Yangtze main stream with regards to the inputs from its major tributaries in order to understand the potential effects from the change in the distribution of the EASM precipitation. For this purpose, we have conducted a systematic sampling of the Yangtze River water to determine the concentration and mineral composition of suspension loads during summer in 2011 and winter in 2012. Water samples were taken at main junctions of the major tributaries. The mineral composition of suspended material were determined by X-ray diffraction (XRD) analyses as well as amount of suspended solids (SS: mg/L) . Abundances of clay minerals relative to quartz tend to decrease from upstream to downstream, while feldspars and calcite increases downstream. Since clay minerals are generally finer than feldspars and calcite, it is suggested that the relatively coarse minerals were removed due to the grain size fractionation during transportation of the suspended particles. Mineral composition in winter is overridden by higher suspension load from upper to middle reaches in summer, which is characterized by higher plagioclase / K-feldspar, higher calcite and dolomite, and wider half-height widths of smectite and illite in summer. Kaolinite is only dominant in the southeastern drainage. These features enable us to distinguish suspended materials from the upper reaches from those from the lower reaches, which could be useful to estimate the dominant area of high precipitation and erosion within the large Yangtze drainage.